Increased demand for high-speed communications services has required that economical and efficient new devices and techniques be developed to support performance increases. For example, as transmission rates climb to the 10-40 Gbps range and beyond in modern optical networks, signal processing and conditioning techniques must be applied to filter out noise and reduce interference such as inter-symbol interference (ISI). Typical optical networks are plagued by noise and bandwidth limitations caused by polarization mode dispersion, modal dispersion, chromatic dispersion, limited component bandwidth, and/or other undesired phenomena. Such effects often cause problems such as group delay distortion, frequency-related attenuation, and/or others. Furthermore, ISI can be time varying due to a variety of causes, such as physical vibration, mechanical stresses, and temperature fluctuations. Typically, optical receivers may use devices such as equalizers to improve the overall performance of such systems and minimize the error rate. However, the implementation of such devices has proven to be challenging and costly.
One particularly difficult problem in the design of optical receivers relates to mutual interference that may exist between an adaptive equalizer and other circuit(s). FIG. 1 is a block diagram of a typical optical receiver 100 capable of receiving signals from an optical communication channel. As shown in the figure, receiver 100 includes a photo diode 102, an adaptive equalizer 104, and a clock recovery circuit 106. Generally speaking, an optical signal is received at optical diode 102, which converts the optical signal into an electrical signal. The electrical signal is provided to adaptive equalizer 104. Adaptive equalizer 104 performs adaptive equalization on the electrical signal to reduce effects of ISI and outputs an equalized signal. Adaptive equalizer interacts with clock recovery circuit 106, which operates to provide symbol timing information to adaptive equalizer 104. Thus, one or more signals may be sent between adaptive equalizer 104 and clock recovery circuit 106. As optical receiver 100 operates, mutual interference may occur between components of adaptive equalizer 104 and components of clock recovery circuit 106. Such interference can be associated with instability affecting one or more components of optical receiver 100 and can lead to dramatic performance degradations.
FIG. 2 is a block diagram of a portion of a receiver containing an adaptive equalizer coupled to a clock recovery circuit that minimizes mean squared error (MMSE). This clock recovery technique allows recovery of timing information from a signal associated with a closed or nearly closed eye diagram, such as that of a signal emerging from a multimode optical fiber channel. As shown in FIG. 2, the adaptive equalizer and clock recovery circuit together comprise a feed-forward filter 202 and a feed-back filter 204, a slicer 206, a slope estimator 208, a low-pass filter 210, and a voltage-controlled oscillator (VCO) 212.
The adaptive equalizer illustrated in FIG. 2 is a decision-feedback equalizer (DFE) that utilizes both feed-forward filter 202 and feed-back filter 204, as well as slicer 206. Feed-forward filter 202 may be a linear transversal filter having taps spaced at a fractional symbol interval T/2. The received signal is provided as input to feed-forward filter 202. For example, the received signal may be the electric signal outputted by photo diode 102 in FIG. 1. Feed-forward filter 202 operates to reduce inter-symbol interference caused by yet-to-be detected symbols, producing a first equalized signal. This first equalized signal is summed with a feedback signal that is the output of feed-back filter 204, to produce a second equalized signal. The first equalized signal is shown as being first sampled at a rate of 2/T, then down-sampled at a rate of 1/T. Timing for this sampling is provided by the clock recovery circuit, described in further detail below. The second equalized signal is provided to slicer 206, which performs a threshold function on the second equalized signal to produce a signal representing detected symbol decisions. The symbol decisions are provided as input to feed-back filter 204. By providing a feedback signal, based on detected symbols, that may be added to the first equalized signal, feed-back filter 204 operates to reduce inter-symbol interference caused by previously detected symbols. The equalizer shown in FIG. 2 is adaptive in the sense that coefficients, or taps, of the feed-forward filter 202 and/or feed-back filter 204 are automatically adjusted to optimize one or more performance measures, such as an error measure. Such adaptation allows the receiver to reduce effects of inter-symbol interference, even when channel conditions are time-varying.
The clock recovery circuit utilizes slope estimator 208, low-pass filter 210, VCO 212, and slicer 206. Slope estimator 208 receives the 2/T sampled version of the first equalized signal produced by the feed-forward filter 202. A slope estimate signal produced by slope estimator 208 is multiplied with an error signal representing the difference between the input of slicer 206 and the output of slicer 206. The resulting signal from this multiply operation is provided to low-pass filter 210. The output of low-pass filter 210 is then used as input to control VCO 212. The output of VCO 212 is used to drive the timing of the sampling operation performed on the first equalized signal. Thus, slope estimator 208, low-pass filter 210, and VCO 212 form parts of a phase lock loop (PLL) that recovers symbol timing for the receiver. This circuit operates by minimizing the mean-squared-error signal representing the difference between the input of slicer 206 and the output of slicer 206. While not explicitly illustrated in the figure, the clock recovery circuit may generate timing signals that are multiples or fractions of the estimated symbol rate. For example, timing signals at twice the symbol rate, one-half the symbol rate, and/or other variations based on the symbol rate, may be generated.
The arrangement shown in FIG. 2 has an advantage over conventional clock recovery circuits that rely on edge information, which can be completely smeared by some severe optical channels, making it difficult to estimate the average zero crossing of the channel output. This arrangement also has an advantage over other well-known clock recovery approaches that involve extracting tones at half the symbol rate and passing such tones through a non-linearity to extract symbol clock information, which may not be applicable to all multimode optical fiber channels due to the possible presence of deep spectral notches at half the symbol rate on particular channels.
Despite these and other desirable properties, the arrangement shown in FIG. 2 is prone to interference that may exist between portions of the adaptive equalizer and portions of the clock recovery circuit. For example, at the same time that taps of feed-forward filter 202 and/or feed-back filter 204 are automatically adjusted to reduce effects of inter-symbol interference, the phase lock loop involving VCO 212 is actively tuning to track the symbol timing. Such dynamic operations interfere with one another, and this mutual interference may cause instability in one, some, or all of the operations involved.
The arrangement in FIG. 2 illustrates interference between an adaptive equalizer and a closely coupled clock recovery circuit. However, the problem of mutual interference is not confined to this specific example. There may be many scenarios in which interference may develop between parts of an adaptive equalizer and one or more other circuits. The effects of such mutual interference may range from minor performance degradations to complete failure of a receiver, as an unstable system. Thus, there is a significant need for effective techniques to reduce mutual interference between an adaptive equalizer and other circuits.